Conventional aircraft exhibit speed stability due to inherent physical characteristics. An aerodynamically stable airframe with stationary control surfaces will tend to maintain a constant angle of attack. The pilot controls the aircraft about the pitch axis by adjusting the angle of a control surface relative to the airstream. This causes the aircraft to fly at different angles-of-attack, which results in aircraft speed changes.
The tendency of an aircraft to fly at a constant angle-of-attack with fixed controls may be termed angle-of-attack stability. Consider now an aircraft in level flight at constant speed and angle-of-attack with the engine thrust, weight, lift and drag forces in balance. Normally, such an aircraft will speed up or slow down, respectively, with forward or aft control stick deflection and will return to the original speed when the control stick is returned its original position. Also, if the thrust is increased slightly, the aircraft will accelerate, causing increased lift and drag. This results in a climb. Eventually, equilibrium conditions consisting of a steady climb rate with the forward speed approximately equal to the initial speed will be reached. These tendencies of an aircraft to return to the initial speed when disturbed are termed speed stability. This speed stability is usually manifested as an oscillatory interchange of potential and kinetic energies called the phugoid mode. If the phugoid mode damping is positive, speed stability exists.
Modern aircraft designs have included inherently unstable aircraft. To stabilize such an aircraft and give it properties similar to an aircraft with inherent aerodynamic stability requires an aircraft motion feedback loop and the use of reliable full authority fly-by-wire (FBW) flight control systems. However, such FBW systems may not exhibit angle-of-attack stability. In addition, typical FBW control systems utilize rate gyros and accelerometers to provide the artificial stability augmentation. Such systems cause the aircraft to hold a constant pitch angular rate and constant normal acceleration, rather than a constant angle-of-attack. In other words, these systems can give excellent handling qualities for maneuvering flight, but do not provide the long-term speed stability of a conventional aircraft. Such lack of speed stability is undesirable in certain circumstances. For example, on approach to landing, the speed of the aircraft has to be low. As a consequence, it is being operated close to stall speed and a further loss of speed could result in a stall. Hence, speed stability is desirable because it requires increasing the back pressure on the pitch control stick when the aircraft slows, in order to maintain a straight, constant flight path. This cues the pilot to the unsafe condition.
A considerable body of prior art which teaches the use of airspeed feedback loops in aircraft control applications exists. However, none of the prior art addresses the speed stability augmentation for full authority FBW flight control systems or for control of unstable airframes. A case in point is U.S. Pat. No. 3,624,364 issued to Dommasch. In there, the use of airspeed feedback operating through a mechanical clutch is disclosed. However, the Dommasch invention appears to be directed to the stabilization of an aircraft when pilot inputs are not present, since the clutch needs to be disengaged if the pilot is to control the movement of the aircraft. Further, the Dommasch system is not intended to function in conjunction with other vehicle motion sensor feedbacks.